1. Field of the Invention
The present invention relates in general to a hydraulically operated brake system of a motor vehicle, and more particularly to an improvement in accuracy of control of the brake pressures for the vehicle wheels to maintain the slip ratios of the wheels within an optimum range.
2. Discussion of the Related Art
An anti-lock brake control device, a wheel traction control device, and a brake control device adapted to provide a desired braking effect are known as an apparatus for controlling the brake pressure for a motor vehicle.
The anti-lock brake control device and the wheel traction control device are designed to regulate the brake pressure so as to prevent an excessive slip ratio of the vehicle wheels upon braking or acceleration of the vehicle. The brake control device adapted to provide a desired braking effect is designed to regulate the brake pressure so that the actual braking effect given to the vehicle coincides with a desired value determined by a brake operating amount such as the operating force applied to a brake pedal or the operating amount of the brake pedal, irrespective of variations in the friction coefficient of friction menders of wheel brakes, amount of load of the vehicle and gradient of the road surface.
An example of such brake pressure control apparatus is disclosed in JP-A-2-310161, which includes (1) wheel speed obtaining means for obtaining the actual rotating speed of a wheel of the vehicle, (2) reference wheel speed obtaining means for obtaining a reference speed of the wheel as a desired wheel speed, on the basis of a running speed of the vehicle, (3) commanding means for generating brake control commands for increasing or decreasing the brake pressure to be applied for braking the wheel, on the basis of the actual wheel speed obtained by the wheel speed obtaining means and the reference wheel speed obtained by the reference wheel speed obtaining means, and (4) pressure regulating means operated according to the brake control commands, to regulate the brake pressure.
The brake pressure is increased or decreased by the pressure regulating means indicated above, which is operated on the basis of the brake control commands. It takes some time after an increase or decrease in the brake pressure, and before a change in the actual wheel speed occurs as a result of the brake pressure increase or decrease.
Thus, the known brake pressure control apparatus suffers from a control delay. In view of this drawback, the commanding means is commonly adapted to generate the brake control commands on the basis of not only the wheel speed but also the wheel acceleration value.
However, the wheel acceleration value is likely to be influenced by external disturbances such as the waviness or bumpiness of the road surface and the vibration of the wheel. The use of a filter for effectively smoothing the wheel acceleration value so as to remove the external disturbances causes a delay in obtaining the wheel acceleration value, and offsets the advantage of introducing the wheel acceleration value for the generation of the brake control commands.
Thus, it has been difficult to suitably generate or prepare the brake control commands, and this has been an obstacle to the improvement in the accuracy of control of the brake pressure.